1. Field of the Invention
The present invention relates to a method for the preparation of a gallium phosphide epitaxial wafer used in the manufacture of green-emitting gallium phosphide light-emitting diodes. More particularly, the invention relates to a method for growth of a nitrogen-doped gallium phosphide epitaxial layer(s) in which an epitaxially grown gallium phosphide layer(s) is doped with nitrogen as an isoelectronic trap.
2. Description of the Related Art
Devices of semiconductor-based light-emitting diode are manufactured usually from a semiconductor epitaxial wafer having a p-n junction obtained by forming a plurality of epitaxially grown semiconductor layers on a semiconductor substrate. For example, a green-emitting gallium phosphide light-emitting diode is obtained by using a gallium phosphide epitaxial wafer prepared by successively forming an n-type gallium phosphide layer and a p-type gallium phosphide layer on an n-type gallium phosphide substrate.
As is described above, an epitaxially grown layer of gallium phosphide, referred to simply as GaP layer hereinafter, is formed on an n-type gallium phosphide substrate, including a multilayered n-type GaP substrate obtained by forming an epitaxially grown layer(s) of n-type GaP in advance on the n-type GaP substrate, by the method of liquid-phase epitaxial growth such as the so-called melt-back liquid-phase epitaxial growth method.
The melt-back liquid-phase epitaxial growth method above mentioned is a method in which a melt of gallium is put on a gallium phosphide substrate followed by temperature elevation, for example, up to 950.degree. C. so as to melt the surface layer of the GaP substrate into the melt of gallium to form a solution, and then the temperature is decreased at a specified rate, for example, down to 800.degree. C. so that the gallium phosphide dissolved in the melt of gallium is precipitated onto the surface of the gallium phosphide substrate to cause epitaxial growth of a GaP layer.
As is known, GaP is an indirect-transition semiconductor which exhibits only a very low light-emitting efficiency even by forming a p-n junction as such. Accordingly, it is usually practiced to enhance the light-emitting efficiency by doping the n-type GaP layer in the vicinity of the p-n junction with nitrogen which serves as an emission center or, namely, an isoelectronic trap. To say particularly, it is practiced to effect the liquid-phase epitaxial growth of an n-type gallium phosphide layer by passing a doping gas containing ammonia NH.sub.3 over the melt of gallium.
As is shown by the chemical reaction equation (1) below, EQU NH.sub.3 (gas)+Ga(liquid).fwdarw.GaN(solid)+3/2H.sub.2 (gas),(1)
gallium nitride GaN is formed in the melt of gallium by the reaction of gallium and ammonia when a doping gas containing ammonia is passed over the melt of gallium. The nitrogen taken into the melt of gallium in the form of gallium nitride serves as a dopant in the n-type GaP layer as the liquid-phase epitaxial growth proceeds.
The light-emitting diode prepared from a gallium phosphide epitaxial wafer having an n-type GaP layer doped with nitrogen in the above described manner, referred to as the GaP(N) light-emitting diode hereinafter, emits a yellowish green light having a peak wavelength of about 567 nm. FIG. 4 is a graph showing the luminous intensity of a GaP(N) light-emitting diode as a function of the concentration of nitrogen in the nitrogen-doped n-type GaP layer.
In the GaP(N) light-emitting diode, the concentration of nitrogen doped in the n-type GaP layer to serve as an isoelectronic trap is an important factor which determines the light-emitting efficiency of the GaP(N) light-emitting diode. As is shown in FIG. 4, the luminous intensity of the GaP(N) light-emitting diode increases as the concentration of nitrogen is increased within the solid-solubility limit of nitrogen in GaP. Accordingly, the nitrogen concentration in an n-type gallium phosphide layer is increased in the prior art by increasing the concentration of ammonia in the gaseous atmosphere for doping.
FIG. 5 shows the relationship between the concentration of ammonia in the gaseous atmosphere and the concentration of nitrogen in the GaP layer grown under the conditions including a growing temperature decreased from 950.degree. C. to 800.degree. C. and a growth rate of 30 .mu.m per hour. As is understood from FIG. 5, the concentration of nitrogen in the n-type GaP layer increases as the concentration of ammonia in the gaseous atmosphere is increased within the range where the concentration of ammonia in the gaseous atmosphere does not exceed about 0.07% by volume. When the concentration of ammonia is further increased to exceed about 0.07% by volume, the concentration of nitrogen in the n-type GaP layer is rather decreased as the concentration of ammonia is increased.
This phenomenon is the reason for the extreme difficulties encountered in the method of controlling the concentration of nitrogen in the GaP layer by the modification of the concentration of ammonia in the gaseous doping atmosphere, when it is desired to control the nitrogen concentration in the GaP layer at a high level within the region of high concentration of ammonia in the gaseous atmosphere.